Intermittent ac and heat vent problems

Hello I am having some problems with my ranger and I was hoping someone might have some advice about it. I have a 2001 ranger 2 wheel drive with the 3.0. The problem I am having is that when I start my truck of the morning everything is fine but in about 30 min heat or air stops coming out of the vents I can hear the air come from behind the dash but it's not really coming out anywhere it's like it's in between settings. I have heard that the default is the defrost when there is a vacuum leak but there isn't really anything coming from there and sometimes it will just start working right again for no reason when I'm going down the road. I checked behind the glove box and didn't notice any bad vacuum lines but I don't have a tester to really do an in depth check on them. Has anyone had a problem like this? Is this a vacuum problem or something else? Thank you for any advice you might have

I know this post is a little old, however, I had a similar problem with my 97 ranger. The re-circ and the fresh air blend door actuator was the issue. The vacuum line was pulling just fine but the actual actuator was malfunctioning. I just tied the blend door with a cable tie to always do the re-circ function. This seemed to help the A/C work the best. I then would run the A/C control on the A/C function, not the max A/C.